IPCC Fourth Assessment Report

• 1. Introduction

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  The IPCC Fourth Assessment Report opened with three words that stopped the world: "Warming of the climate system is unequivocal." Published in 2007 under the title Climate Change 2007, this was the fourth in a series of reports by the United Nations Intergovernmental Panel on Climate Change, and by every measure it was the most ambitious scientific undertaking of its kind. More than 2,500 scientific expert reviewers contributed. More than 800 authors helped draft it. Over 450 lead authors shaped its conclusions. People from over 130 countries had a hand in its production, and the whole effort took six years to complete. The report drew on over 6,000 peer-reviewed scientific studies.

  What the report delivered was not merely a catalogue of alarming findings. It was a reckoning: a document designed to tell governments, in precise and defensible language, exactly how certain scientists were about what was happening to the planet, who was responsible, and what choices remained. The questions it planted were stark. How much warming was already locked in, regardless of what humanity did next? What would different futures look like, depending on the path taken? And what would it cost, in money and in lives, to act or not to act? The answers were spread across four working group reports, a Synthesis Report, and a set of findings that would shape climate policy for years to come.

• 2. The Science of Warming

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  Carbon dioxide measured at 379 parts per million in 2005 had burst far beyond anything the Earth had seen in the previous 650,000 years, when the natural range sat between 180 and 300 parts per million. Working Group I, which covered the physical science, was produced by 676 authors from 40 countries and reviewed by over 625 expert reviewers. Its Summary for Policymakers went through something unusual before publication: every line was reviewed in person by representatives of 113 governments during the tenth session of Working Group I, held from January to February 2007.

  The warming signal the report documented was clear. Eleven of the twelve years between 1995 and 2006 ranked among the twelve warmest in the instrumental record going back to 1880. Over the preceding century, global average temperature had risen by about 0.74 degrees Celsius, up from the 0.6 degrees recorded over the comparable period reported in the Third Assessment. Arctic temperatures had climbed at nearly twice the global average rate over that same hundred-year span.

  The oceans told an equally striking story. Since 1961, the ocean had absorbed more than 80 percent of the heat added to the climate system, with measurable warming reaching depths of at least 3,000 meters. Sea level had risen at an average rate of about 1.8 millimeters per year between 1961 and 2003, then accelerated to about 3.1 millimeters per year between 1993 and 2003, though scientists could not yet determine whether that acceleration was a long-term shift or natural variability.

  On the question of cause, the Summary for Policymakers was direct: most of the observed increase in global average temperatures since the mid-20th century was very likely due to observed increases in human-caused greenhouse gas concentrations. In the report's terminology, "very likely" meant an assessed probability exceeding 90 percent. That framing, grounded in expert judgment rather than casual language, became one of the document's most quoted features.

• 3. What the Models Projected

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  Six families of scenarios drove the projections in the Fourth Assessment Report, each built around different assumptions about economic growth, energy use, and international cooperation. The scenarios had been established in 2000 through the Special Report on Emissions Scenarios, and the models that ran them represented the best available tools for projecting 21st-century climate. Best-estimate temperature rises by 2090-2099, relative to 1980-1999 levels, ranged from 1.8 degrees Celsius under the most optimistic scenario to 4.0 degrees under the most pessimistic.

  Even in the most optimistic case, the report projected a temperature rise of about 0.1 degrees Celsius per decade for the next two decades, even if greenhouse gas and aerosol concentrations were frozen at year 2000 levels. That number reflected warming already committed to by past emissions. Under all scenarios, the projected pace doubled to about 0.2 degrees per decade over the same near-term window.

  Sea level projections were deliberately conservative. Working from models that excluded ice sheet flow because of gaps in published literature, scientists estimated rises of 18 to 38 centimeters in a low scenario and 26 to 59 centimeters in a high scenario by century's end. The report itself flagged this as potentially an underestimate, noting that dynamic processes related to ice flow not included in current models, but suggested by recent observations, could increase the vulnerability of ice sheets and push actual sea level rise higher.

  For the Arctic, the picture was especially sharp. Some model projections showed Arctic late-summer sea ice nearly disappearing by the latter part of the century. Climate sensitivity, defined as the warming expected from a doubling of carbon dioxide, was placed at a likely range of 2 to 4.5 degrees Celsius, with a best estimate of about 3 degrees.

• 4. Impacts Across Continents

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  Working Group II, released in its Summary for Policymakers on the 6th of April 2007, drew on evidence from all continents and most oceans to show that natural systems were already responding to regional climate changes. With very high confidence, meaning about a nine-in-ten chance of being correct, it concluded that spring events such as the unfolding of leaves, egg-laying, and migration were happening earlier, and that plant and animal species were shifting their ranges toward the poles and to higher altitudes.

  Fresh water systems faced a bifurcating future. Annual average river runoff was projected with high confidence to increase by 10 to 40 percent at high latitudes and in some wet tropical areas, while decreasing by 10 to 30 percent over dry regions at mid-latitudes and in the dry tropics. Glaciers and snowpack storing water for hundreds of millions of people were projected to shrink across the century.

  For coastal areas, the Working Group projected with very high confidence that sea surface temperature increases of about 1 to 3 degrees Celsius would result in more frequent coral bleaching events and widespread mortality, unless corals developed thermal adaptation. Many millions more people were projected to be flooded every year due to sea-level rise by the 2080s.

  On biodiversity, the Synthesis Report placed the stakes plainly. Roughly 20 to 30 percent of species assessed faced increased extinction risk if global average warming exceeded 1.5 to 2.5 degrees Celsius relative to 1980-1999 levels. At warming beyond about 3.5 degrees Celsius, model projections pointed toward significant extinctions affecting 40 to 70 percent of species assessed globally. These figures were offered with medium confidence, reflecting both the strength of the signal and the genuine difficulty of projecting biological responses across such diverse systems.

• 5. The Cost of Acting and Not Acting

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  Working Group III convened in Bangkok on the 30th of April 2007, bringing together over 400 scientists and experts from about 120 countries to draft its Summary. By the full IPCC meeting on the 4th of May, agreement was reached among some 2,000 delegates on a document that addressed the economics of climate mitigation in concrete terms.

  One of the most striking conclusions was how modest the cost of action appeared relative to the cost of inaction. Stabilizing atmospheric greenhouse gas concentrations at between 445 and 535 parts per million, the most ambitious target discussed, was projected to reduce average annual GDP growth rates by less than 0.12 percent. Stabilizing at a slightly higher range of 535 to 590 parts per million would reduce annual GDP growth rates by 0.1 percent. The report noted that a substantial fraction of those costs could be offset by health benefits from reduced air pollution, improved energy security, and increased agricultural production in some regions.

  For electricity generation, the report projected that renewable energy could supply 30 to 35 percent of global electricity by 2030, up from 18 percent in 2005, at a carbon price of up to 50 US dollars per tonne. Nuclear power was projected to rise modestly from 16 percent to 18 percent over the same period. The report warned that higher oil prices might drive exploitation of high-carbon alternatives such as oil sands and synthetic fuels from coal, undoing gains unless carbon capture and storage technologies were deployed.

  The report also flagged a troubling trend in public investment. Government funding in real absolute terms for most energy research programs had been flat or declining for nearly 20 years and stood at about half the 1980 level. Carbon prices of 5 to 65 US dollars per tonne by 2030, rising to 15 to 130 US dollars per tonne by 2050, were identified as consistent with stabilization around 550 parts per million by 2100.

• 6. Politics, Negotiations, and Errors

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  The scientific process that produced the Fourth Assessment Report ran directly into political pressure at several turns. Patricia Romero Lankao, a lead author from the National Center for Atmospheric Research, reported that US negotiators succeeded in removing language from the Working Group II report calling for cuts in greenhouse gas emissions. A sentence stating that mitigation measures would be required alongside adaptation simply disappeared from the final version.

  China objected to wording describing very high confidence that natural systems on all continents and in most oceans were being affected by regional climate changes. When China asked that the word "very" be removed, three scientific authors refused to agree, and the deadlock was broken only by deleting any reference to confidence levels in that passage.

  US Energy Secretary Samuel Bodman said at a news conference that the report was "sound science" and that human activity contributing to climate change was no longer up for debate. Lord Rees, the president of the Royal Society, said the report made it clearer than ever before that human actions were written large on the observed changes in climate. French President Chirac led a "Paris Call for Action" signed by 46 countries calling for a new United Nations Environment Organization modeled on the more powerful World Health Organization. The United States, China, Russia, and India, the top four greenhouse gas emitters, notably did not join.

  The report was not without error. A projection in Working Group II suggested that Himalayan glaciers could disappear by 2035. The IPCC later acknowledged the date was incorrect, while maintaining that the summary-level conclusion about Himalayan glaciers remained robust. The error traced back through a misquotation chain: the IPCC had correctly quoted a World Wildlife Fund report, which had itself misquoted an International Commission on Snow and Ice report. Former IPCC chairman Robert Watson said the mistakes appeared to have gone in the direction of overstating impact, while Martin Parry, who had co-chaired Working Group II, argued that what began as one unfortunate error had become what he called "a clamour without substance".

• 7. The Synthesis and Its Legacy

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  A draft Synthesis Report was published on the 16th of November 2007, designed to compress the three working group reports into a single readable document aimed at policymakers. Its timing was deliberate: the 13th Conference of the Parties in Bali, Indonesia, had been postponed to December specifically so the Synthesis could appear first.

  The Synthesis Report drew six themes across its pages, from observed changes and their causes to long-term adaptation and mitigation strategies. Among its most forward-pointing conclusions was the finding on committed warming: both past and future carbon dioxide emissions would continue to drive warming and sea level rise for more than a millennium, given the timescales required for removing that gas from the atmosphere. Adaptation alone, the Synthesis made clear, could not cope with all projected effects of climate change over the long run.

  Several science academies issued joint statements citing AR4 conclusions in 2007, 2008, and 2009, including the academies of Brazil, China, India, Mexico, South Africa, and the G8 nations. The Inter Academy Medical Panel statement, signed by 43 scientific academies, followed in 2010. The Netherlands Environmental Assessment Agency carried out two independent reviews and found them generally supportive of the report's conclusions.

  In September 2010, the United Nations appointed an independent board of scientists to review the workings of the IPCC in light of the Himalayan glacier episode and other criticisms. The US National Research Council, also in 2010, concluded that climate change was occurring, was caused largely by human activities, and posed significant risks across a broad range of human and natural systems, explicitly citing the Fourth Assessment Report as one of the assessments consistent with that conclusion. The Synthesis Report's final discussion of robust findings and key uncertainties would feed directly into the work of scientists already preparing what would become the Fifth Assessment Report.